1. Field of the Invention
The present invention relates to a pretensioner integrated with a webbing take-up device in which a vehicle occupant-restraining webbing is wound on a winding shaft in a layered form, and rotating the winding shaft by a predetermined amount in a direction in which the webbing is taken up by a rack and pinion type driving mechanism instantaneously with rapid deceleration of a vehicle.
2. Description of the Related Art
There has conventionally been used a so-called pretensioner which causes a winding shaft to rotate by a predetermined amount in a direction in which a webbing is taken up instantaneously with rapid deceleration of a vehicle so as to improve an ability of restraining a vehicle occupant by a webbing.
Various types of driving mechanism of such a pretensioner have been examined and put to practical use. A pretensioner using a rack and pinion type driving mechanism which is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-156751 will be described below.
In the pretensioner disclosed therein, a cylindrical pipe is mounted to a housing provided at,a side portion of a webbing take-up device. A hollow piston is inserted into the cylindrical pipe so as to be movable in an axial direction of the pipe, and gunpowder and an ignition device are disposed within the piston. Further, rack teeth are formed at a predetermined position on the outer periphery of the piston and along the axial direction of the piston, and the piston itself functions as a rack bar.
A notch is formed at a predetermined position on the cylindrical pipe in the axial direction of the pipe, and pinion teeth of a pinion comes into the cylindrical pipe from the notch so as to be engageable with the rack teeth. A first gear wheel whose diameter is larger than that of the pinion is coaxially connected to the pinion, and the first gear wheel engages with a second gear wheel. The second gear wheel is integrated with a coupling wheel which functions as a clutch. The second gear wheel is connected to an axial end of the winding shaft of the webbing take-up device via the coupling wheel.
In the above-described structure, when a vehicle decelerates rapidly, gunpowder fires by the ignition device, and thrust generated during the ignition causes the piston (rack bar) to move within the cylindrical pipe in the axial direction. As a result, the pinion engaging with the rack teeth of the piston rotates around its axis so as to rotate the first gear wheel and the second gear wheel. The coupling wheel connected to the axial end of the winding shaft is integrated with the second gear wheel, and therefore, when the second gear wheel rotates, the winding shaft is rotated via the coupling wheel in the direction in which the webbing is taken up.
However, in the above-described structure, not only the pinion but also intermediate gears such as the first gear wheel and the second gear wheel are interposed between the piston (rack bar) serving as a driving starting point and the winding shaft serving as a driving end point, and therefore, a space for installation of these intermediate gears becomes necessary in the housing of the pretensioner. Accordingly, there arises a problem in that the pretensioner be as a whole made larger both in the radial and axial directions thereof.
In view of the above-described facts, an object of the present invention is to provide a pretensioner which can be made smaller as a whole.
A first aspect of the present invention is a pretensioner which is integrated with a webbing take-up device in which a vehicle occupant-restraining webbing is wound on a winding shaft, and at the time of rapid deceleration of a vehicle, which rotates the winding shaft by a predetermined amount in a direction in which the webbing is taken up by using a rack and pinion type driving mechanism, wherein the driving mechanism comprises: a piston disposed so as to be movable in a cylinder in an axial direction thereof and including a rack portion provided with rack teeth, the piston being provided to move at the time of rapid deceleration of the vehicle; and a pinion disposed so as to engage with the rack teeth and directly connected to an axial end of the winding shaft via a clutch which. is connected to the winding shaft, only at the time of rapid deceleration of the vehicle.
In accordance with a second aspect of the present invention, in the structure provided by the first aspect, the pinion includes a toothed portion in which pinion teeth engaging with the rack teeth are formed, and a clutch portion formed coaxially and integrally with the toothed portion and including the clutch at the side of an inner periphery thereof.
In accordance with a third aspect of the present invention, in the structure provided by the second aspect, the piston includes a piston portion to which gas pressure is applied, and the rack portion is made upright at a position offset from the center of the piston portion, the rack portion being disposed so as to move within a range of dimension of an outer diameter of the clutch portion.
In accordance with a fourth aspect of the present invention, in the structure provided by the second aspect, the rack portion is disposed so that a total length, in a predetermined direction, of the toothed portion and the rack portion in an engaged state is less than or equal to a dimension of an outer diameter of the clutch portion, the predetermined direction being a direction substantially perpendicular to a direction of a rotating shaft of the pinion and to a direction. to which the rack portion moves.
According to the first aspect of the present invention, when a vehicle decelerates rapidly, the pretensioner integrated with the webbing take-up device operates. Specifically, the piston (including the rack portion provided with rack teeth), which is disposed within the cylinder and is formed as a rack bar, moves in an axial direction of the piston. When the piston moves in the axial direction, the pinion disposed so as to engage with the rack teeth of the piston is rotated. The pinion provided in the present invention is directly connected to an axial end of the winding shaft via the clutch which is connected to the winding shaft only at the time of rapid deceleration of the vehicle. Therefore, when the pinion rotates, the winding shaft is rotated via the clutch in the direction in which the webbing is taken up. As a result, the vehicle occupant-restraining webbing is instantaneously taken up by an amount corresponding to a moving stroke of the piston and an ability of restraining a vehicle occupant by a webbing is thereby improved.
In the present invention, since the pinion which rotates due to driving force of the piston formed as the rack bar is directly connected to the axial end of the winding shaft via the clutch, intermediate gears as used in a conventional system become unnecessary. For this reason, it is not necessary that a space of installation for the intermediate gears be provided in the pretensioner. Accordingly, the pretensioner can as a whole be made smaller both in axial and radial directions thereof.
According to the second aspect of the present invention, the pinion is structured so as to include the toothed portion and the clutch portion which are integrated with each other. Therefore, as compared with a case in which the toothed portion and the clutch portion are separately provided, the space of installation can be further reduced and the number of parts can also be reduced.
According to the third aspect of the present invention, the piston includes the piston portion, and the rack portion is made upright at a position offset from the center of the piston portion and is also disposed so as to move in an axial direction of the piston within the range of dimension of the outer diameter of the clutch portion. As a result, a rack and pinion type driving mechanism can be prevented from being made larger outwardly in the radial direction of the clutch portion.